Apparatus of the above-mentioned type are known in themselves and are described in detail for example in the German "Offenlegungsschrift" No. 25 23 803 and U.S. Pat. No. 3,585,131. It may be said, therefore, that these documents constitute part of the background art. For further illustration of the same, reference is made to U.S. Pat. Nos. 3,411,630, 3,501,011, 3,511,381, 3,516,548, 3,734,298, 3,837,496, 4,051,041, 4,062,778, and 4,113,625 all of which show apparatus comprising a stack of spacer plates with membranes preferably arranged in pairs between them which, however, cannot be said to be folded in zigzag. Such folding is disclosed instead, for example, in the U.S. Pat. Nos. 4,116,841, 3,929,295, 3,862,031, 3,788,482 and 3,780,870.